In order to increase the power output and efficiency of an internal combustion engine, some engine power packs use an air compressor to deliver compressed air to the intake of the engine. Engines provided with such a system are typically referred to as forced induction engines. Engines that do not have forced inductions are typically referred to as naturally aspirated engines.
One example of an air compressor is a turbocharger. In a turbocharger, the exhaust gases from the engine turn a turbine which compresses the air to be supplied to the engine. In a turbocharged engine, the desired compression pressure is reached when the combination of engine speed and throttle angle provide sufficient exhaust flow to the turbine and therefore the necessary energy for the compressor to deliver the air intake flow to the engine at the desired pressure.
The pressure ratio at the compressor (i.e. the ratio of the output air pressure of the compressor over the input air pressure of the compressor) is an important parameter to monitor in order to avoid surging.
Compressor surge is a condition which is reached when the engine's air consumption is suddenly interrupted while the compressor supplies a large flow of compressed air. For example, when the engine is operated with the throttle valve wide open, a lot of air is supplied to the engine. Should the user release the throttle operator completely, as shown in FIG. 7, the throttle valve closes to its idle position, as shown in FIG. 8. An example of throttle operator is a throttle pedal. Even though the throttle valve closes almost immediately, the components of the air compressor have inertia and will take some time to reduce their speeds. This condition where a high pressure ratio in conjunction with a low air flow creates pressure surges that can potentially damage the air compressor. These air pressure surges also generate noise that can be heard and which can be a nuisance to a user of a vehicle having the engine.
The conditions that can potentially damage an air compressor due to pressure surge vary from one compressor to the other. As such, the technical specifications of most air compressors include a graph illustrating a surge limit line. The graph illustrates the surge limit line in terms of flow rate versus pressure ratio. FIG. 5 illustrates such a graph for an exemplary compressor. In this graph, the horizontal axis corresponds to the air flow rate and the vertical axis corresponds to the pressure ratio P3/P2, where P3 is the air pressure downstream of the air compressor and P2 is the air pressure upstream of the air compressor. The surge limit line is shown as a dashed line. When the air compressor operates under the conditions corresponding to the region to the right of the surge limit line, no damage to the air compressor should occur due to pressure surge. When the air compressor operates under the conditions corresponding to the region to the left of the surge limit line, damage to the air compressor due to pressure surge may occur.
In FIG. 5, the dash-dot line illustrates a case where the throttle valve is wide open and is then suddenly closed to its idle position as shown in FIG. 8. When the throttle valve is wide open, the air compressor operates at the rightmost point of the dash-dot line. When the throttle valve is closed, the flow rate decreases and the pressure ratio initially increases before decreasing as can be seen by following the arrows on the dash-dot line. As can be also seen, the dash-dot line crosses the dashed surge limit line and extends to the left thereof. As a result, the operating conditions of the air compressor could cause pressure surge to damage the air compressor.
In order to address the issue of damage and noise resulting from pressure surge, many engines are provided with a bypass valve upstream of the throttle valve. In such implementations, when the pressure upstream of the throttle valve becomes too high, the bypass valve opens thereby releasing some of the pressure. From the bypass valve, the air is exhausted to the atmosphere or returned upstream of the air compressor. The dotted line in FIG. 5 illustrates a case where the throttle valve is wide open and is then suddenly closed completely where a bypass valve is provided. As can be seen, the dotted line does not cross the surge limit line, and the compressor is therefore unlikely to suffer damage due to pressure surge.
Although the bypass valve helps reduce the likelihood of damage to the air compressor and noise due to air pressure surge, the bypass valve adds weight, cost and assembly complexity to the power pack in which it is provided.
It would therefore be desirable to provide a solution to the problem of damage caused to air compressors due to air pressure surge and to the problem of the noise generated due to air pressure surge that does not require a bypass valve.